libkvm: add kvm_walk_pages API.

This API allows callers to enumerate all known pages, including any
direct map & kernel map virtual addresses, physical addresses, size,
offset into the core, & protection configured.

For architectures that support direct map addresses, also generate pages
for any direct map only addresses that are not associated with kernel
map addresses.

Fix page size portability issue left behind from previous kvm page table
lookup interface.

Reviewed by: jhb
Sponsored by: Backtrace I/O
Differential Revision: https://reviews.freebsd.org/D12279